In the offshore industry, Floating Production Storage and Offloading (FPSO's) vessels have in the past years continuously increased in size and complexity. Topsides are provided with oil and/or gas processing equipment that includes steam turbine electrical generators, condensate export pumps, oil cracking installations, distillation equipment, heat exchangers, gas to liquid (GTL) plants, LNG, liquid petroleum gas (LPG) or mixed hydrocarbon production, liquefaction and processing equipment. The last three generations of FPSO's have seen an increase in topside weight from 1000 tons to over 20,000 tons. These heavy and high topsides have a negative effect on the vessel's stability while deck space is limited in view of the dense layout of the equipment placed on deck. Also storage capacity within the hull reaches its limits.
In U.S. Pat. No. 7,101,118 a twin-hull construction is shown in which two hulls are interconnected via bracings, and a single wide deck structure is fitted across the hulls, processing equipment or a fuel powered power generator being placed on the deck structure. Hydrocarbons can be stored in tanks, which may comprise cryogenic LNG tanks, in either hull prior to offloading via shuttle tankers, before and after processing. The hulls may be converted oil or LNG tankers, with their storage tanks for hydrocarbons situated completely within the hulls or partly extending above deck level such as in case of spherical LNG tanks. A turret, moored to the sea bed is placed between the hulls.
In DE 27 07 628 a twin-hull vessel is described constructed from two existing hulls that are interconnected via a bracing structure, and a single external hull surrounding the existing hulls. Personnel quarters, LNG liquefaction equipment, transfer means and a flare tower are provided on the deck. A single point mooring arrangement, moored to the sea bed and carrying a production riser is placed in the forward deck structure.
The above multi hull structures have increased weight carrying capacity and can accommodate large and complex process installations such as GTL, LPG, LNG or a mix of hydrocarbon related processes. The twin hull structures have improved stability and provide a relatively large storage capacity. Furthermore, they can be used in combination with large size turrets, increasing the area of application. Also, they are able to operate at lower drafts to provide a high freeboard and allow dry tree usage.
The above known multi hull vessels have as a disadvantage that a wide overlying deck structure is placed over both interconnected hulls onto which overlying deck structure the processing equipment is later mounted. The large overlying deck is formed by a heavy and complex steel structure. This requires a large construction site for accommodating the combined hulls during mounting of the processing equipment, involving expensive and large sized dry-docks.
It is hence an object of the present invention to provide a multi hull vessel of increased storage capacity and weight carrying capacity, allowing installation of process equipment for oil production, gas treating, gas liquefaction, oil refining, oil cracking, gas to liquid conversion and other hydrocarbon processes. It is a further object to provide a floating structure carrying processing equipment on the deck structure, which can be constructed easily and in a cost effective manner.